The whole purpose of a software application is to provide valuable services to its consumers. These valuable services can be constructed from features.

In such a way every backwards-compatible change to the application can be one of the next points:
1) adding a new feature (technical or business request)
2) fixing a bug for the previously added feature

If the modification is not done in a backwards-compatible manner then we can think about the software as a new application. This idea is supported by semantic versioning: MAJOR.MINOR.PATCH

The software from a consumer point of view is seen as a presentation of parallel and independent feature blocks:
On the other side coding corresponds to text, and text updating is a sequential operation: some symbols are replaced, some are added, removed or just moved across the text. In addition to this the code from the small "Feature a" often intersects with the small "Feature b" without having strong separation. Thus coding leads to the united and sequential picture of a software, where every bit is appended to the end of a common block:
We can think of it as a process of book writing, with all the chapters and the references between its different parts. The idea of a simultaneous text editing by several authors is a source of lots of problems. Let us specify our first problem

Contradiction (1)As a writer I would like to change a piece of a text without affecting other editors of the same text.

Contradiction (1 strengthen)As a writer I need to apply changes to a text that is no longer exist (moved, changed or removed).A text should be changed and should not be changed.
The simple solution is to apply golden Divide and Conquer method and totally eliminate simultaneous editing problem.Solution (1)Divide text blocks in such a manner that every writer has its own independent part.
Books are often divided by chapters where there is no more than one author per chapter.
Applications can be divided in the same manner with a developer being assigned to support a specific software library. Removing the multi-user editing mode can be also done by usage of small services, for example, when one developer is responsible for a bunch of related cloud functions.

Files renaming, moving files across the project, spacious code refactoring, all these steps lead to later painful code merging. If Solution 1 is not accessible then there are some weak solutions, which do not solve the problem but minimize its pain.

Weak solution (1a)Divide people in small groups, so the teams can provide autonomous and independent deployable services. This approach is called usage of Microservices. We can minimize the impact of a huge sharable code base by presenting small groups of people, which manage small independent micro projects with small code bases. Also the team becomes primarily responsible for the service business logic and will have deeper understanding of the processes than large teams do.

As long as a code merging is inevitable we also can reduce the pain of conflicts resolving by minimizing applied changes.Weak solution (1b)
The team can make a rule: to commit only small changes to sharable code base and do it often, at least every day. Thus the developers will have only small merge conflicts, which must be simple to resolve.

This weak solution actually leads us to a new contradictionContradiction (2)As a developer sometimes I work on a large task during several days. As a developer I have to make small commits to the common code base at least every day, so the other developers can simply merge my changes.In other words: changes should be large and be applied rarely, changes should be small and be applied often.

Solution (2)Wrap large changes in a switchable toggle. Make this toggle 'off' by default. Commit small changes to the common code base for the disabled feature. Simply enable the feature when it is ready.
This is a very common principle in software building, which is called Feature Toggle. Now a developer is able to do small and often changes to a common code base.

Sometimes there is a need to do a breaking change that affects a large group of people. For example, a change in the logging library that is used across the whole application; or take, for instance, a field renaming in a live database. If we check the problem carefully then the contradiction is the same as the contradiction (1):Contradiction (3)A part of something should be changed and at the same time should not be changed.

One of the solution is to make long-lasting switch to a new breakable change. I call this principle 3-phase switching:Solution (3)1) Create the enhanced version of the block. If there is a sharable state (database, cache, external API) then the new version should also support the old state presentation. 2) Allow or force clients to switch to the new version of the block during some time.3) Stay with the new version and remove the old block; or stay with the old solution (rollback).
For example, if a developer makes a breakable change in the logging library like method renaming or drastically change method parameters, then he or she can create a new class (a new library) based on the old one. So there will be two blocks: the old one and the new one. The consumers can slowly switch to the new library.

The concept of 3-phase switching is very powerful and is used widely in different aspects:

Canary release, which is a process of rolling up a new software to a small group of users and checking whether it works as expected, thus not affecting all customers by a big bang of failure. Rolling up is slowly moving: [10% of customers redirected to the new software, 90% to the old], [20% to new, 80% to the old], ..., [100% to the new software].

Let's take another example with a table field renaming in a relational database. The process might look as:

1) Add the new field with a proper name to the table.2) Create a new endpoint for clients (API) in such a manner that data is written to both: the new and the old field. Wait for clients being switched to the new version.Due the fact that the new API writes data in compatible way to the old field, clients that are using the old field are not affected at all.

3) When there are no more clients using the old API then simply remove it with the old field. Finally there will be only new renamed field.